This application is based on Japanese Patent Application No. 2001-153718 filed on May 23, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a zoom lens system, and particularly to a zoom lens system suitable as a projection optical system for use in a projector in which an image formed by, for example, a digital micromirror device (hereinafter referred to as a xe2x80x9cDMDxe2x80x9d) serving as a display device is projected onto a screen.
2. Description of the Prior Art
These days, compact, high-performance projectors employing a DMD as a display device are rapidly becoming widespread for the purposes of presentation at meetings, viewing of BS (broadcast satellite) digital broadcasts and commercially available movie software in households (the so-called home theater), and the like. As display devices become increasingly compact and high-resolution through the adaptation of DMDs, zoom lens systems for projection are required to be accordingly compact and high-performance. However, it is generally believed that a zoom lens system for projecting an image formed by a DMD needs to have a long back focal length to permit the insertion of a TIR (total internal reflection) prism for separating light beams. In addition, such a zoom lens system needs to be compact but satisfactorily corrected for various aberrations (such as lateral chromatic aberration), and is often required to be wide-angle to offer a large screen in a narrow room.
For use in projectors, various types of five-unit zoom lens system have been proposed. For example, Japanese Patent Application Laid-Open No. H11-101940 proposes a zoom lens system composed of a negative, a positive, a positive, a negative, and a positive lens unit. Here, the second to fourth lens units move during zooming, with the fourth lens unit moving toward the screen during zooming from the telephoto end to the wide-angle end. U.S. Pat. No. 6,137,638 proposes a zoom lens system composed of a negative, a positive, a positive, a negative, and a positive lens unit. Here, the second and third lens units move toward the display device and the fourth lens unit moves toward the screen during zooming from the telephoto end to the wide-angle end. Japanese Patent Application Laid-Open No. H11-190821 proposes a zoom lens system composed of a negative, a positive, a negative, a positive, and a positive lens unit. Here, mainly the second to fourth lens units move during zooming. Japanese Patent Application Laid-Open No. 2000-137165 proposes a zoom lens system composed of a negative, a positive, a positive, a negative, and a positive lens unit. Here, the second to fourth lens units move during zooming, with the fourth lens unit moving together with an aperture stop toward the reduction side during zooming from the telephoto end to the wide-angle end. Japanese Patent Application Laid-Open 2000-292701 proposes a zoom lens system composed of a negative, a positive, a positive, a negative, and a positive lens unit. Here, the second to fourth lens units move during zooming, with the fourth lens unit, which is composed of a single negative meniscus lens element, moving toward the reduction side during zooming. Moreover, Japanese Patent Application Laid-Open 2001-108900 proposes a six-unit zoom lens system composed of a negative, a positive, a positive, a negative, a positive, and a positive lens unit. Here, an aperture stop is provided in the third lens unit, and the second, third, and fifth lens units move during zooming.
All of these conventional examples are zoom lens systems designed chiefly to project an image formed by a liquid crystal display device, and therefore they are compact, indeed, but are insufficiently corrected for distortion, lateral chromatic aberration, and other aberrations, and in addition have a short back focal length. For example, the zoom lens system proposed in Japanese Patent Application Laid-Open No. H11-101940 is unsatisfactory in the correction of distortion, high-order lateral chromatic aberration, astigmatic difference, and other aberrations. The zoom lens system proposed in U.S. Pat. No. 6,137,638, despite its unimpressive zoom ratio of 1.25xc3x97, suffers from large variation of distortion accompanying zooming and large astigmatic difference.
The zoom lens system proposed in Japanese Patent Application Laid-Open No. H11-190821 is unsatisfactory in the correction of lateral chromatic aberration, distortion, and other aberrations, and is unsuitable for use with a compact, high-resolution display device such as a DMD. Another problem is that, since the fourth lens unit has a high positive optical power and an aperture stop is provided in the second lens unit, which is the main zoom unit, increasing the zoom ratio makes it difficult to maintain a telecentric construction. The zoom lens system proposed in Japanese Patent Application Laid-Open No. 2000-137165 suffers from large astigmatic difference and is unsatisfactory in the correction of distortion and lateral chromatic aberration. The zoom lens system proposed in Japanese Patent Application Laid-Open 2000-292701 achieves the correction of lateral chromatic aberration by using four or five lens elements made of high-refractive-index, high-dispersion glass out of the total 13 constituent lens elements. Such glass, however, exhibits poor coloring properties, and making heavy use of it leads to low transmittance to blue light. Thus, this type of glass is unsuitable for use in projection optical systems that are expected to offer high-quality images. Moreover, this zoom lens system, despite its unimpressive zoom ratio, suffers from large variation of distortion accompanying zooming.
The zoom lens system proposed in Japanese Patent Application Laid-Open 2001-108900 has the disadvantage of large lateral chromatic aberration and distortion. It suffers also from large astigmatic difference, and is thus unsuitable for use in projection optical systems that employ a DMD as a display device. Another problem is that, since the fifth lens unit has a high positive optical power and the aperture stop moves together with the third lens unit, it is difficult to match the front-side focal length position of the rear lens unit, i.e. the lens system formed by all the lens elements arranged to the rear of the aperture stop, with the aperture position during zooming. Still another problem is that, since the first to fourth lens units have high negative optical powers and the overall construction is not quite afocal, it is difficult, with this construction, to obtain a long back focal length while maintaining satisfactory off-axial performance.
An object of the present invention is to provide a compact zoom lens system having a long back focal length and satisfactorily corrected for various aberrations.
To achieve the above object, according to the present invention, a zoom lens system is provided with, from an enlargement side, a first lens unit having a negative optical power, a second lens unit having a positive optical power and disposed on the image side of the first lens unit with a first variable distance secured in between, a third lens unit having a positive optical power and disposed on the image side of the second lens unit with a second variable distance secured in between, a fourth lens unit having a negative optical power, disposed on the image side of the third lens unit with a third variable distance secured in between, and including an aperture stop, a fifth lens unit having a positive or negative optical power and disposed on the image side of the fourth lens unit with a fourth variable distance secured in between, and a sixth lens unit having a positive optical power and disposed on the image side of the fifth lens unit with a fifth variable distance secured in between. Here, zooming is achieved by varying the first to fifth variable distances. Moreover, the third and fifth lens units move from the enlargement side to the reduction side during zooming from the telephoto end to the wide-angle end. Moreover, the zoom lens system is substantially telecentric toward the reduction side.